Polyimide resins for high temperature wear applications

ABSTRACT

Polyimide resin compositions that contain an aromatic polyimide, graphite and a sepiolite filler, or a mixture of a sepiolite filler and a kaolin filler, are found to exhibit low wear and high thermal oxidative stability. Such compositions are especially useful in molded articles that are exposed to wear conditions at high temperatures such as aircraft engine parts.

TECHNICAL FIELD

This disclosure relates to filled polyimide resin compositions that are useful for high temperature wear applications such as aircraft engine parts.

BACKGROUND

The unique performance of polyimide compositions under stress and at high temperatures have made them useful in applications requiring high wear resistance, particularly at conditions of high pressure and velocity. Some examples of such applications are aircraft engine parts, aircraft wear pads, automatic transmission bushings and seal rings, tenter frame pads and bushings, material processing equipment parts, and pump bushings and seals.

Typically, a polyimide component in applications as described above is intended to function as a sacrificial, or consumable, component, thereby preventing or reducing the wear or damage that a more costly mating or adjacent component would experience if it were mated against some other component. However, as the polyimide component wears, the resulting increased clearances can result in other adverse effects, such as increased leakage (of air pressure or fluid) or increased noise, thereby reducing the operating effectiveness of the entire system in which the polyimide component is contained. Restoring the system to its original operating effectiveness would require replacement of the worn polyimide component with a new un-used polyimide component. Replacement may require disassembly, reassembly, testing and re-calibration (“service”) of the system, resulting in considerable costs in terms of down-time and labor. Thus, a polyimide component that demonstrates a lower rate of wear is desirable to reduce the frequency of replacement and service, thereby reducing cost.

Despite the variety of polyimide compositions, and additives for those compositions such as graphite, that have previously been available, a need still remains for polyimide compositions that exhibit as molded parts the desirably high degree of wear resistance at high temperatures required for applications such aircraft engine parts, while maintaining the other advantageous attributes of the polyimide material.

SUMMARY

In one embodiment, this invention provides composition that includes an aromatic polyimide, graphite and a sepiolite filler or a mixture of a sepiolite filler and a kaolin filler.

In another embodiment, this invention provides composition that includes (a) about 40 weight parts or more and yet about 54 weight parts or less of an aromatic polyimide, (b) about 46 weight parts or more and yet about 60 weight parts or less graphite, and (c) about 0.5 weight parts or more and yet about 3.0 weight parts or less of a sepiolite filler; where all weight parts combined together total to 100 weight parts.

Yet another embodiment of the invention hereof is a composition of matter substantially as shown or described in any one or more of FIGS. 1˜4.

Articles fabricated from the above described compositions are also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and/or embodiments of this invention are illustrated in drawings as described below. These features and/or embodiments are representative only, and the selection of these features and/or embodiments for inclusion in the drawings should not be interpreted as an indication that subject matter not included in the drawings is not suitable for practicing the invention, or that subject matter not included in the drawings is excluded from the scope of the appended claims and equivalents thereof.

FIG. 1 is a schematic representation of the experimental design space used in the examples.

FIG. 2 is a response surface plot of the thermal oxidative stability model derived in the examples.

FIG. 3 is a response surface plot for the resin wear model derived in the examples.

FIG. 4 is a superposition of the response surface plots for resin wear and for thermal oxidative stability as derived in the examples.

DETAILED DESCRIPTION

Disclosed herein are compositions that contain (a) an aromatic polyimide, (b) graphite, and (c) a sepiolite filler or a mixture of a sepiolite filler and a kaolin filler.

A polyimide as used as the component “(a)” in a composition hereof is polymer in which at least about 80%, preferably at least about 90%, and more preferably essentially all (e.g. at least about 98%) of the linking groups between repeat units are imide groups. An aromatic polyimide as used herein includes an organic polymer in which about 60 to about 100 mol %, preferably about 70 mol % or more, and more preferably about 80 mol % or more of the repeating units of the polymer chain thereof have a structure as represented by the following Formula (I):

wherein R¹ is a tetravalent aromatic radical and R² is a divalent aromatic radical, as described below.

A polyimide polymer suitable for use herein may be synthesized, for example, by reacting a monomeric aromatic diamine compound (which includes derivatives thereof) with a monomeric aromatic tetracarboxylic acid compound (which includes derivatives thereof), and the tetracarboxylic acid compound can thus be the tetracarboxylic acid itself, the corresponding dianhydride, or a derivative of the tetracarboxylic acid such as a diester diacid or a diester diacidchloride. The reaction of the aromatic diamine compound with an aromatic tetracarboxylic acid compound produces the corresponding polyamic acid (“PAA”), amic ester, amic acid ester, or other reaction product according to the selection of starting materials. An aromatic diamine is typically polymerized with a dianhydride in preference to a tetracarboxylic acid, and in such a reaction a catalyst is frequently used in addition to a solvent. A nitrogen-containing base, phenol or an amphoteric material can be used as such a catalyst.

A polyamic acid, as a precursor to a polyimide, can be obtained by polymerizing an aromatic diamine compound and an aromatic tetracarboxylic acid compound, preferably in substantially equimolar amounts, in an organic polar solvent that is generally a high-boiling solvent such as pyridine, N-methylpyrrolidone, dimethylacetamide, dimethylformamide or mixtures thereof. The amount of all monomers in the solvent can be in the range of about 5 to about 40 wt %, in the range of about 6 to about 35 wt %, or in the range of about 8 to about 30 wt %, based on the combined weight or monomers and solvent. The temperature for the reaction is generally not higher than about 100° C., and may be in the range of about 10° C. to 80° C. The time for the polymerization reaction generally is in the range of about 0.2 to 60 hours.

Imidization to produce the polyimide, i.e. ring closure in the polyamic acid, can then be effected through thermal treatment, chemical dehydration or both, followed by the elimination of a condensate (typically, water or alcohol). For example, ring closure can be effected by a cyclization agent such as pyridine and acetic anhydride, picoline and acetic anhydride, 2,6-lutidine and acetic anhydride, or the like.

In various embodiments of the thus-obtained polyimide, about 60 to 100 mole percent, preferably about 70 mole percent or more, more preferably about 80 mole percent or more, of the repeating units of the polymer chain thereof have a polyimide structure as represented by the following Formula (I):

wherein R¹ is a tetravalent aromatic radical derived from the tetracarboxylic acid compound; and R² is a divalent aromatic radical derived from the diamine compound, which may typically be represented as H₂N—R²—NH₂.

A diamine compound as used to prepare a polyimide for a composition hereof may be one or more of the aromatic diamines that can be represented by the structure H₂N—R²—NH₂, wherein R² is a divalent aromatic radical containing up to 16 carbon atoms and, optionally, containing one or more (but typically only one) heteroatoms in the aromatic ring, a heteroatom being, for example, selected from —N—, —O—, or —S—. Also included herein are those R² groups wherein R² is a biphenylene group. Examples of aromatic diamines suitable for use to make a polyimide for a composition hereof include without limitation 2,6-diaminopyridine, 3,5-diaminopyridine, 1,2-diaminobenzene, 1,3-diaminobenzene (also known as m-phenylenediamine or “MPD”), 1,4-diaminobenzene (also known as p-phenylenediamine or “PPD”), 2,6-diaminotoluene, 2,4-diaminotoluene, and benzidines such as benzidine and 3,3′-dimethylbenzidine. The aromatic diamines can be employed singly or in combination. In one embodiment, the aromatic diamine compound is 1,4-diaminobenzene (also known as p-phenylenediamine or “PPD”), 1,3-diaminobenzene (also known as m-phenylenediamine or “MPD”), or mixtures thereof.

Aromatic tetracarboxylic acid compounds suitable for use to prepare a polyimide for a composition hereof may include without limitation aromatic tetracarboxylic acids, acid anhydrides thereof, salts thereof and esters thereof. An aromatic tetracarboxylic acid compound may be as represented by the general Formula (II):

wherein R¹ is a tetravalent aromatic group and each R³ is independently hydrogen or a lower alkyl (e.g. a normal or branched C₁˜C₁₀, C₁˜C₈, C₁˜C₆ or C₁˜C₄) group. In various embodiments, the alkyl group is a C₁ to C₃ alkyl group. In various embodiments, the tetravalent organic group R¹ may have a structure as represented by one of the following formulae:

Examples of suitable aromatic tetracarboxylic acids include without limitation 3,3′,4,4′-biphenyltetracarboxylic acid, 2,3,3′,4′-biphenyltetracarboxylic acid, pyromellitic acid, and 3,3′,4,4′-benzophenonetetracarboxylic acid. The aromatic tetracarboxylic acids can be employed singly or in combination. In one embodiment, the aromatic tetracarboxylic acid compound is an aromatic tetracarboxylic dianhydride, particularly 3,3′,4,4′-biphenyltetracarboxylic dianhydride (“BPDA”), pyromellitic dianhydride (“PMDA”), 3,3,4,4′-benzophenonetetracarboxylic dianhydride, or mixtures thereof.

In one embodiment of a composition hereof, a suitable polyimide polymer may be prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (“BPDA”) as the aromatic tetracarboxylic acid compound, and from greater than 60 to about 85 mol % p-phenylenediamine (“PPD”) and 15 to less than 40 mol % m-phenylenediamine (“MPD”) as the aromatic diamine compound. Such a polyimide is described in U.S. Pat. No. 5,886,129 (which is by this reference incorporated as a part hereof for all purposes), and the repeat unit of such a polyimide may also be represented by the structure shown generally in the following Formula (III):

wherein greater than 60 to about 85 mol % of the R² groups are p-phenylene radicals:

and 15 to less than 40 mol % are m-phenylene radicals:

In an alternative embodiment, a suitable polyimide polymer may be prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (“BPDA”) as a dianhydride derivative of the tetracarboxylic acid compound, and 70 mol % p-phenylenediamine and 30 mol % m-phenylenediamine as the diamine compound.

A polyimide as used herein is preferably a rigid polymer. A polyimide polymer is considered rigid when there are no, or an insignificant amount (e.g. less than 10 mol %, less than 5 mol %, less than 1 mol % or less than 0.5 mol %) of, flexible linkages in the polyimide repeating unit. Flexible linkages are moieties that are predominantly composed of a small number of atoms, and that have an uncomplicated structure (such as straight-chain rather than branched or cyclic), and thus permit the polymer chain to bend or twist with relative ease at the location of the linkage. Examples of flexible linkages include without limitation: —O—, —N(H)—C(O)—, —S—, —SO₂—, —C(O)—, —C(O)—O—, —C(CH₃)₂—, —C(CF₃)₂—, —(CH₂)—, and —NH(CH₃)—. Although disfavored, these or other flexible linkages, when present, are sometimes found in the R² portion of an aromatic diamine compound.

A polyimide as used herein is preferably an infusible polymer, which is a polymer that does not melt (i.e. liquefy or flow) below the temperature at which it decomposes. Typically, parts prepared from a composition of an infusible polyimide are formed under heat and pressure, much like powdered metals are formed into parts (as described, for example, in U.S. Pat. No. 4,360,626, which is by this reference incorporated as a part hereof for all purposes).

A polyimide as used herein preferably has a high degree of stability to thermal oxidation. At elevated temperature, the polymer will thus typically not undergo combustion through reaction with an oxidant such as air, but will instead vaporize in a thermolysis reaction.

Graphite is used as the component “(b)” of a composition hereof. Graphite is typically added to a polyimide composition to improve wear and frictional characteristics, and to control the coefficient of thermal expansion (CTE). The amount of graphite used in a polyimide composition for such purpose is thus sometimes advantageously chosen to match the CTE of the mating components.

Graphite is commercially available in a variety of forms as a fine powder, and may have a widely varying average particle size that is, however, frequently in the range of from about 5 to about 75 microns. In one embodiment, the average particle size is in the range of from about 5 to about 25 microns. In another embodiment, graphite as used herein contains less than about 0.15 weight percent of reactive impurities, such as those selected from the group consisting of ferric sulfide, barium sulfide, calcium sulfide, copper sulfide, barium oxide, calcium oxide, and copper oxide.

Graphite as suitable for use herein can be either naturally occurring graphite or synthetic graphite. Natural graphite generally has a wide range of impurity concentrations, while synthetically produced graphite is commercially available having low concentrations of reactive impurities. Graphite containing an unacceptably high concentration of impurities can be purified by any of a variety of known treatments including, for example, chemical treatment with a mineral acid. Treatment of impure graphite with sulfuric, nitric or hydrochloric acid, for example, at elevated or reflux temperatures can be used to reduce impurities to a desired level.

A sepiolite filler, or a mixture of a sepiolite filler and a kaolin filler, is used as the component “(c)” of a composition hereof. A sepiolite filler suitable for use herein includes sepiolite itself [Mg₄Si₆O₁₅(OH)₂.6(H₂O)], which is a hydrated magnesium silicate filler that exhibits a high aspect ratio due to its fibrous structure. Unique among the silicates, sepiolite is composed of long lath-like crystallites in which the silica chains run parallel to the axis of the fiber. The material has been shown to consist of two forms, an α and a β form. The α form is known to be long bundles of fibers and the β form is present as amorphous aggregates.

A sepiolite filler suitable for use herein also includes attapulgite (also known as palygorskite), which is almost structurally and chemically identical to sepiolite except that attapulgite has a slightly smaller unit cell.

A sepiolite filler suitable for use herein also includes clays that are layered fibrous materials in which each layer is made up of two sheets of tetrahedral silica units bonded to a central sheet of octahedral units containing magnesium ions [see, e.g., FIGS. 1 and 2 in L. Bokobza et al, Polymer International, 53, 1060-1065 (2004)]. The fibers stick together to form fiber bundles, which in turn can form agglomerates. These agglomerates can be broken apart by industrial processes such as micronization or chemical modification (see, e.g., European Patent 170,299 to Tolsa S. A.).

In one embodiment, a sepiolite filler suitable for use herein includes a rheological grade sepiolite clay, such as that which is described in EP-A-454,222 and/or EP-A-170,299 and marketed under the Pangel® trademark by Tolsa S. A., Madrid, Spain. The term “rheological grade” in this context refers to a sepiolite clay typically having an average surface area greater than 120 m²/g [as measured in N₂ by the Brunauer/Emmett/Teller method (as described in Brunauer et al, “Adsorption of Gases in Multimolecular Layers”, Journal of the American Chemical Society, 60: 309-19, 1938)], and typically having average fiber dimensions of about 200 to 2000 nm long, 10-30 nm wide, and 5-10 nm thick. Rheological grade sepiolite is obtained from natural sepiolite by means of micronization processes that substantially prevent breakage of the sepiolite fibers, such that the sepiolite disperses easily in water and other polar liquids, and has an external surface with a high degree of irregularity, a high specific surface, greater than 300 m²/g and a high density of active centers for adsorption, that provide it a very high water retaining capacity upon being capable of forming, with relative ease, hydrogen bridges with the active centers. The microfibrous nature of the rheological grade sepiolite particles makes sepiolite a material with high porosity and low apparent density.

Additionally, rheological grade sepiolite has a very low cationic exchange capacity (10-20 meq/100 g) and the interaction with electrolytes is very weak, which in turn causes rheological grade sepiolite to not be practically affected by the presence of salts in the medium in which it is found, and therefore, it remains stable in a broad pH range. The above-mentioned qualities of rheological grade sepiolite can also be found in rheological grade attapulgite, which typically has a particle size smaller than 40 microns, such as the range of ATTAGEL® clays (for example ATTAGEL 40 and ATTAGEL 50) manufactured and marketed by Engelhard Corporation, United States; and the MIN-U-GEL range of products from Floridin Company.

A kaolin filler suitable for use herein includes kaolinite itself, which is a sheet-type silicate whose molecules are arranged in two sheets or plates, one of silica and one of alumina. Kaolinite is a clay mineral with the chemical composition Al₂Si₂O₅(OH)₄. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra. Rocks that are rich in kaolinite are known as china clay or kaolin. In contrast, smectites such as montmorillonite clay minerals are arranged in two silica sheets and one alumina sheet. The molecules of the smectites are less firmly linked together than those of the kaolinite group and are thus further apart. Maintaining the phase stability of crystal structure of the sheet silicates is desirable, as is maintaining the thermal stability of the structural water of the sheet silicates at higher temperatures, such as up to about 450° C. [as shown, for example, by thermogravimetric analysis (TGA)]. Loss of structural water during processing of a polyimide composition can result in harm to polyimide integrity, and possibly change the crystal structure of the sheet silicate, giving a harder, more abrasive compound. Examples of sheet silicates that are not stable enough to be included in the compositions described herein are montmorillonite, vermiculite, and pyrophyllite.

Sepiolite fillers and kaolin fillers suitable for use herein are discussed further in Murray, Applied Clay Science 17(2000) 207-221. When a mixture of a sepiolite filler and a kaolin filler is used as the component (c) in a composition of this invention, each type of filler may be present in the mixture in an amount in the range of about 10 wt % to about 90 wt %, based on the total weight of all fillers together in the mixture.

The graphite, and the sepiolite filler (or the mixture of a sepiolite filler and a kaolin filler), as used in the compositions and articles hereof are frequently incorporated into the heated solvent prior to transfer of the PAA polymer solution (or other solution for other types of monomers) as described above, so that the resulting polyimide is precipitated in the presence of the components (b) and (c), which thereby become incorporated into the composition.

In the compositions of this invention, the content of the various components includes all of the possible ranges that may be formed from the following amounts:

-   -   component (a), an aromatic polyimide, may be present in an         amount of about 40 weight parts or more, about 42 weight parts         or more, about 44 weight parts or more or about 46 weight parts         or more, and yet in an amount of about 54 weight parts or less,         about 52 weight parts or less, about 50 weight parts or less or         about 48 weight parts or less;     -   component (b), a graphite, may be present in an amount of about         46 weight parts or more, about 48 weight parts or more, about 50         weight parts or more or about 52 weight parts or more, and yet         in an amount of about 60 weight parts or less, about 58 weight         parts or less, about 56 weight parts or less or about 54 weight         parts or less; and     -   component (c), a sepiolite filler or a mixture of a sepiolite         filler and a kaolin filler, may be present in an amount of about         0.5 weight parts or more, about 0.75 weight parts or more, about         1.0 weight parts or more, about 1.25 weight parts or more or         about 1.5 weight parts or more, and yet in an amount of about         3.0 weight parts or less, about 2.75 weight parts or less, about         2.5 weight parts or less, about 2.25 weight parts or less or         about 2.0 weight parts or less.         In a composition hereof, the amounts of the respective weight         parts of the three components as combined together in any         particular formulation, taken from the ranges as set forth         above, will total to 100 weight parts.

The compositions of this invention include all of the formulations in which the compositional content may be expressed by any combination of the various maxima and minima, as set forth above, for any one component of the composition together with any such combination of maxima and minima for either or both of the other two components.

One or more additives may be used as an optional component “(d)” of a composition hereof. When used, additive(s) may be used in an amount in the range of about 5 to about 70 wt % based on the total weight of all four components together in a 4-component [(a)+(b)+(c)+(d)] composition, with the total weight of three components together in a 3-component [(a)+(b)+(c)] composition being in the range of about 30 to about 95 wt % based on the total weight of all four components together in a 4-component [(a)+(b)+(c)+(d)] composition.

Additives suitable for optional use in a composition hereof may include, without limitation, one or more of the following: pigments; antioxidants; materials to impart a lowered coefficient of thermal expansion, e.g. carbon fibers; materials to impart high strength properties e.g. glass fibers, ceramic fibers, boron fibers, glass beads, whiskers, graphite whiskers or diamond powders; materials to impart heat dissipation or heat resistance properties, e.g. aramid fibers, metal fibers, ceramic fibers, whiskers, silica, silicon carbide, silicon oxide, alumina, magnesium powder or titanium powder; materials to impart corona resistance, e.g. natural mica, synthetic mica or alumina; materials to impart electric conductivity, e.g. carbon black, silver powder, copper powder, aluminum powder or nickel powder; materials to further reduce wear or coefficient of friction, e.g. boron nitride or poly(tetrafluoroethylene) homopolymer and copolymers. Fillers may be added as dry powders to the final resin prior to parts fabrication.

Materials suitable for use in or to make a composition hereof may themselves be made by processes known in the art, or are available commercially from suppliers such as Alfa Aesar (Ward Hill, Mass.), City Chemical (West Haven, Conn.), Fisher Scientific (Fairlawn, N.J.), Sigma-Aldrich (St. Louis, Mo.) or Stanford Materials (Aliso Viejo, Calif.).

As with products made from other infusible polymeric materials, parts fabricated from a composition hereof may be made by techniques involving the application of heat and pressure (see, for example, U.S. Pat. No. 4,360,626). Suitable conditions may include, for example, pressures in the range of from about from 50,000 to 100,000 psi (345 to 690 MPa) at ambient temperatures. Physical properties of articles molded from a composition hereof can be further improved by sintering, which may typically be performed at a temperature in the range of from about 300° C. to about 450° C.

Parts and other articles prepared from a composition hereof are useful as aircraft engine parts such as bushings, bearings, washers, seal rings, gaskets, wear pads and slide blocks. These parts may be used in all types of aircraft engines such as reciprocating piston engines and, particularly, jet engines. Parts and other articles prepared from a composition hereof are also useful in the following: automotive and other types of internal combustion engines; other vehicular subsystems such as exhaust gas recycle systems and clutch systems; pumps; non-aircraft jet engines; turbochargers; aircraft subsystems such as thrust reversers, nacelles, flaps systems and valves; materials processing equipment such as injection molding machines; material handling equipment such as conveyors, belt presses and tenter frames; and films, seals, washers, bearings, bushings, gaskets, wear pads, seal rings, slide blocks and push pins and other applications where low wear is desirable. In some applications, a part or other article prepared from a composition hereof is in contact with metal at least part of the time when the apparatus in which it resides is assembled and in normal use.

EXAMPLES

The advantageous attributes and effects of the compositions hereof may be seen in a series of examples (Examples 1˜14), as described below. The embodiments of these compositions on which the examples are based are representative only, and the selection of those embodiments to illustrate the invention does not indicate that materials, components, reactants, ingredients, formulations or specifications not described in these examples are not suitable for practicing the inventions herein, or that subject matter not described in these examples is excluded from the scope of the appended claims and equivalents thereof. The significance of the examples is better understood by comparing the results obtained therefrom with the results obtained from certain trial runs that are designed to serve as controlled experiments (Controls A˜D) and provide a basis for such comparison since the compositions therein do not contain any sepiolite (or sepiolite/kaolin mixture) filler.

In the examples, the following abbreviations are used: “BPDA” is defined as 3,3′,4,4′-biphenyltetracarboxylic anhydride, “MPD” is defined as m-phenylenediamine, “PPD” is defined as p-phenylenediamine, “COF” is defined as coefficient of friction, “TOS” is defined as thermal oxidative stability, “avg” is defined as average or mean, “h” is defined as hour(s), “mL” is defined as milliliter(s), “m” is defined as meter, “cm” is defined as centimeter(s), “mm” is defined as millimeter(s), “in” is defined as inch, “g” is defined as gram(s), “kg” is defined as kilogram(s), “oz” is defined as ounce, “psia” is defined as pound per square inch (absolute), “rpm” is defined as revolutions per minute, and “wt %” is defined as weight percent(age).

Materials

3,3′,4,4′-biphenyltetracarboxylic anhydride was obtained from Mitsubishi Gas Chemical Co., Inc. (Tokyo, Japan). M-phenylenediamine and p-phenylenediamine were obtained from DuPont (Wilmington, Del., USA). The graphite used was a synthetic graphite, maximum 0.05% ash, with a median particle size of about 8 microns. Pangel® S-9 sepiolite was purchased from EM Sullivan Associates, Inc. (Paoli, Pa., USA), a distributor for the manufacturer, Tolsa S. A. (Madrid 28001, Spain). Pangel® S-9 sepiolite is a rheological grade of sepiolite the particles of which have an unmodified or uncoated surface.

Methods

Dried polyimide resin was fabricated into tensile bars by direct forming according to ASTM E8 (2006), “Standard Tension Test Specimen for Powdered Metal Products—Flat Unmachined Tensile Test Bar”, at room temperature and 100,000 psi (690 MPa) forming pressure. The tensile bars were sintered at 405° C. for 3 hours with a nitrogen purge.

High temperature wear on the tensile bars was measured at 800° F. (427° C.). In these tests, a steel ball bearing was rubbed against the surface of a test specimen under a 2 pound load for a 3 hour period. At the end of the experiment, the volume of the resulting wear scar on the test specimen (“Resin Wear”) was measured, as was the wear experienced by the steel ball (“Ball Wear”), and the coefficient of friction (“COF”) between the test specimen and the steel ball. Resin Wear was measured by optical profilometry, from which the volume of the wear scar may be determined. The result for both Resin Wear and Ball Wear is reported as the volume of weight lost, stated in in³ or cm³. The result reported for COF is a unitless number as it is a relative coefficient of friction of each piece in relation to the other. All measurements were made using the test procedures described in ASTM G 133-05 (2005), “Standard Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear”, modified by using a temperature controlled oven, with acquisition of friction force data on a computer.

Thermal Oxidative Stability (“TOS”) was tested by first weighing tensile bars, then exposing two pieces of each tensile bar (“TOS-1” and “TOS-2”) to a temperature of 800° F. (427° C.) for a period of 25 hours at a pressure of 88 psia (0.61 MPa) in air. A final weight measurement was then taken, and the percent weight loss of each piece of tensile bar was calculated according to the following formula:

${\%\mspace{14mu}{weight}\mspace{14mu}{loss}} = {\frac{{Initial}\mspace{14mu}{{Wt}.{- {{Wt}.\mspace{14mu}{After}}}}}{{Initial}\mspace{14mu}{{Wt}.}} \times 100}$ and the percentage calculated and reported was percent weight loss. Percent weight loss for TOS-1 and TOS-2 was then averaged.

Examples 1 and 2, Control A

Particles of polyimide resins based on 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), m-phenylene diamine (MPD) and p-phenylene diamine (PPD) containing 50 wt % graphite and 0, 5, or 10 wt % sepiolite filler were prepared according to the method described in U.S. Pat. No. 5,886,129, which is by this reference incorporated in its entirety as a part hereof for all purposes. After drying, the resins were ground through a 20 mesh screen using a Wiley mill. Test specimen tensile bars were then prepared as described above, and Resin Wear, Ball Wear and COF were determined according to the methods described above. The results are shown in Table 1.

TABLE 1 Composition: Weight % based on Polyimide + Resin Wear Ball Wear Example/ Graphite + Sepiolite in 10⁻⁸ in³ in 10⁻⁸ in³ Control Polyimide Graphite Sepiolite (10⁻⁷ cm³) (10⁻⁷ cm³) COF 1 45 50 5 1845 (3023) 2 (3.3)  0.06 2 40 50 10 2225 (3646) 7 (11)  <0.01 A 50 50 0 2275 ± 307 41 ± 5 0.06 ± 0.00 (3728 ± 503) (67 ± 8)

Examples 3˜14, Controls B˜D

These experiments were conducted to examine the effects of compositional content (relative amounts of polymer, graphite and sepiolite filler) on the properties of parts molded from the compositions, including thermal oxidative stability (“TOS”) and Resin Wear.

An extreme vertices design of degree two was used to represent the content and performance relationship of twelve formulations having different compositional content of the three components: polyimide, graphite and sepiolite filler. The design space is summarized as follows, and is illustrated in FIG. 1:

Polyimide 0.30 to 0.50 weight fraction Graphite 0.50 to 0.70 weight fraction Sepiolite 0.00 to 0.10 weight fraction The compositions of Controls B-D represent the condition of 0.00 weight fraction sepiolite since that component was absent therefrom.

Each composition was synthesized and test specimens were prepared as described for Examples 1˜2. TOS and Resin Wear were measured in the manner as described above. The results are summarized in Table 2

TABLE 2 Component Resin Wear Example/ Weight Fraction TOS-1 TOS-2 Average in 10⁻⁸ in³ Control Polyimide Graphite Sepiolite in % in % TOS (10⁻⁷ cm³)  3 0.45 0.5 0.05 4.06 4.04 4.05 1845 (3023)  4 0.375 0.575 0.05 3.87 4.07 3.97 2050 (3359)  5 0.375 0.575 0.05 6.03 6.29 6.16 1435 (2351)  6 0.3875 0.5375 0.075 9.33 8.79 9.06 2050 (3359) B 0.5 0.5 0 2.67 2.54 2.61 2275 (3728)  7 0.4 0.5 0.1 6 6.1 6.05 2050 (3359)  8 0.375 0.575 0.05 6.17 6.24 6.21 1640 (2687) C 0.4 0.6 0 4.38 4.04 4.21 2460 (4031)  9 0.4375 0.5375 0.025 2.13 2.1 2.12 1435 (2352) 10 0.3375 0.5875 0.075 6.36 6.29 6.33 2460 (4031) 11 0.3 0.6 0.1 5.76 5.7 5.73 2665 (4367) 12 0.3375 0.6375 0.025 3.39 3.18 3.29 1435 (2352) 13 0.35 0.55 0.1 8.4 8.29 8.35 2820 (4621) D 0.3 0.7 0 2.2 2.08 2.14 3000 (4916) 14 0.3 0.65 0.05 5.19 5.21 5.20 2050 (3359)

In FIG. 2, contour lines representing a range of average TOS values from 4˜9 are overlaid on the diagram of the design space to produce a surface plot indicating the approximate compositional content that yields a particular average TOS in a molded part. From FIG. 2 it may be seen that, within the range of compositions studied, the content of polyimide had less effect than the content of the other components. Higher levels of graphite improved (i.e. lowered) TOS somewhat, while higher levels of sepiolite filler worsened (i.e. increased) TOS somewhat. In FIG. 2, the lowest TOS is generally in a region around the line representing a TOS of 4, which represents compositions having varying amounts, respectively, of polyimide and graphite.

In FIG. 3, contour lines representing a range of Resin Wear from 1750 to 6000×10⁻⁸ in³ are overlaid on the diagram of the design space to produce a surface plot indicating the approximate compositional content that yields a particular amount of Resin Wear in a molded part. FIG. 3 indicates that there is a minimum in Resin Wear near the midpoint of the sepiolite filler weight fractions in the study. Resin Wear was improved (lowered) somewhat for higher content values of the polyimide, and worsened (increased) somewhat for higher content values of graphite. In FIG. 3, the lowest wear is generally in a region centered approximately on the line where the content of sepiolite filler is 0.05 weight fraction.

FIG. 4 is a superposition of the response surface plots for TOS from FIG. 2 and for Resin Wear from FIG. 3. The line with medium dashes represents the approximate location of the contour representing a TOS of 3.5 or less, and the lines with small dashes represent the approximate location of the contour representing Resin Wear of 2000 or less. The hatched area represents the intersection of those two regions.

Where a range of numerical values is recited herein, the range includes the endpoints thereof and all the individual integers and fractions within the range, and also includes each of the narrower ranges therein formed by all the various possible combinations of those endpoints and internal integers and fractions to form subgroups of the larger group of values within the stated range to the same extent as if each of those narrower ranges was explicitly recited. Where a range of numerical values is stated herein as being greater than a stated value, the range is nevertheless finite and is bounded on its upper end by a value that is operable within the context of the invention as described herein. Where a range of numerical values is stated herein as being less than a stated value, the range is nevertheless bounded on its lower end by a non-zero value.

In this specification, unless explicitly stated otherwise or indicated to the contrary by the context of usage, where an embodiment of the subject matter hereof is stated or described as comprising, including, containing, having, being composed of or being constituted by or of certain features or elements, one or more features or elements in addition to those explicitly stated or described may be present in the embodiment. An alternative embodiment of the subject matter hereof, however, may be stated or described as consisting essentially of certain features or elements, in which embodiment features or elements that would materially alter the principle of operation or the distinguishing characteristics of the embodiment are not present therein. A further alternative embodiment of the subject matter hereof may be stated or described as consisting of certain features or elements, in which embodiment, or in insubstantial variations thereof, only the features or elements specifically stated or described are present.

In this specification, unless explicitly stated otherwise or indicated to the contrary by the context of usage,

-   -   (a) amounts, sizes, ranges, formulations, parameters, and other         quantities and characteristics recited herein, particularly when         modified by the term “about”, may but need not be exact, and may         also be approximate and/or larger or smaller (as desired) than         stated, reflecting tolerances, conversion factors, rounding off,         measurement error and the like, as well as the inclusion within         a stated value of those values outside it that have, within the         context of this invention, functional and/or operable         equivalence to the stated value;     -   (b) all numerical quantities of parts, percentage or ratio are         given as parts, percentage or ratio by weight;     -   (c) use of the indefinite article “a” or “an” with respect to a         statement or description of the presence of an element or         feature of this invention, does not limit the presence of the         element or feature to one in number; and     -   (d) the words “include”, “includes” and “including” are to be         read and interpreted as if they were followed by the phrase         “without limitation” if in fact that is not the case. 

1. An admixed composition characterized in that it possesses thermal oxidative stability and wear resistance, which consists of (a) about 40 weight parts to about 54 weight parts of an aromatic polyimide, (b) about 46 weight parts to about 60 weight parts graphite, and (c) 0.5 weight parts to 2.0 weight parts of a rheological grade sepiolite filler; where all weight parts combined together total to 100 weight parts.
 2. A composition according to claim 1 wherein the aromatic polyimide is prepared from an aromatic tetracarboxylic acid compound or derivative thereof, wherein the aromatic tetracarboxylic acid compound is represented by the Formula (II):

wherein R1 is a tetravalent aromatic group, and each R3 is independently hydrogen or a C1˜C10alkyl group, or mixtures thereof.
 3. A composition according to claim 1 wherein the aromatic polyimide is prepared from an aromatic tetracarboxylic acid compound selected from the group consisting of 3,3′,4,4′-biphenyltetracarboxylic acid, 2,3,3′,4′-biphenyltetracarboxylic acid, pyromellitic acid, and 3,3′,4,4′-benzophenonetetracarboxylic acid, or derivative thereof, or mixtures thereof.
 4. A composition according to claim 1 wherein the aromatic polyimide is prepared from a diamine compound represented by the structure H2N—R2—NH2, wherein R2 is a divalent aromatic radical containing up to 16 carbon atoms and, optionally, containing in the aromatic ring one or more heteroatoms selected from the group consisting of —N—, —O—, and —S—.
 5. A composition according to claim 1 wherein the aromatic polyimide is prepared from a diamine compound selected from the group consisting of 2,6-diaminopyridine, 3,5-diaminopyridine, 1,2-diaminobenzene, 1,3-diaminobenzene, 1,4-diaminobenzene, 2,6-diaminotoluene, 2,4-diaminotoluene, benzidine and 3,3′-dimethylbenzidine.
 6. A composition according to claim 1 wherein the aromatic polyimide comprises the recurring unit

wherein R2 is selected from the group consisting of p-phenylene radicals,

m-phenylene radicals,

and a mixture thereof.
 7. A composition according to claim 6 wherein greater than 60 to about 85 mol % of the R2 groups comprise p-phenylene radicals, and about 15 to less than 40 mol % comprise m-phenylene radicals.
 8. A composition according to claim 6 wherein about 70 mol % of the R2 groups comprise p-phenylene radicals and about 30 mol % of the R2 groups comprise m-phenylene radicals.
 9. A composition according to claim 1 that consists of (a) about 42 weight parts to about 52 weight parts of an aromatic polyimide, (b) about 48 weight parts to about 58 weight parts graphite, and (c) 0.75 weight parts to 2.0 weight parts of a sepiolite filler; where all weight parts combined together total to 100 weight parts.
 10. A composition according to claim 1 wherein a sepiolite filler comprises particles having an unmodified or uncoated surface.
 11. An article fabricated from a composition according to claim
 1. 12. An article according to claim 11 which comprises an internal combustion engine part.
 13. An article according to claim 11 which comprises an aircraft part.
 14. An article according to claim 11 which comprises an automotive part.
 15. An article according to claim 11 which comprises a bushing, bearing, washer, seal ring, wear pad or slide block.
 16. An article according to claim 11 which comprises a part for a gas recycle system; a clutch system; a pump; a turbocharger; a thrust reverser, a nacelle, a flaps system; an injection molding machine; a conveyor, belt press; and a tenter frame. 